Add and use numcopiesneeded preferred content expression.

* Add numcopiesneeded preferred content expression.
* Client, transfer, incremental backup, and archive repositories
  now want to get content that does not yet have enough copies.

This means the asssistant will make copies of files that don't yet
meet the configured numcopies, even to places that would not normally want
the file.

For example, if numcopies is 4, and there are 2 client repos and
2 transfer repos, and 2 removable backup drives, the file will be sent
to both transfer repos in order to make 4 copies. Once a removable drive
get a copy of the file, it will be dropped from one transfer repo or the
other (but not both).

Another example, numcopies is 3 and there is a client that has a backup
removable drive and two small archive repos. Normally once one of the small
archives has a file, it will not be put into the other one. But, to satisfy
numcopies, the assistant will duplicate it into the other small archive
too, if the backup repo is not available to receive the file.

I notice that these examples are fairly unlikely setups .. the old behavior
was not too bad, but it's nice to finally have it really correct.

.. Almost. I have skipped checking the annex.numcopies .gitattributes
out of fear it will be too slow.

This commit was sponsored by Florian Schlegel.

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